In large vehicles such as buses and trucks, foot brakes (friction brakes) are used as principal brakes, and engine brakes or exhaust brakes are used as auxiliary brakes. In recent years, as the engines installed in vehicles have increasingly small displacement, the capacity of engine brakes and exhaust brakes has decreased, resulting in a greater number of cases in which auxiliary brakes have been enhanced by introducing eddy-current devices.
Recently, many such eddy-current devices no longer require current to be applied when braking because they use a permanent magnet as a magnetic pole (e.g., Patent Reference 1). At present, many of these permanent-magnet eddy-current deceleration devices are of the single-row rotation type or the double-row rotation type.
Among these, FIG, 6 illustrates an example of the structure of the single-row rotation type.
In FIG. 6, Reference Numeral 2 is a supporting member formed from a non-magnetic material such as aluminum which is immobilized and supported in a bearing case 1, and which supports a magnet support ring 4 so that it freely rotates via a bearing 3. On the peripheral surface of this magnet support ring 4, a plurality of permanent magnets 6 form an arc having the same radius in a cross-section taken in a direction perpendicular to the center of the rotational axis 5, and the upper and lower magnetic polar surfaces thereof are fixed at equal intervals on the same circumference. In addition, a plurality of pole pieces 7 formed from strong magnets are arranged at equal intervals on the same circumference via a supporting member 8 of a non-magnetic material, and affixed integrally to the supporting member 2 so as to face the outer surfaces of the group of permanent magnets 6. A rotor 9 fits into the rotational axis 5, and a cylindrical portion 9a thereof is caused to face the pole piece 7 so that it has a specified gap, and an actuator is mounted on the circumference in order to rotate the magnet support ring 4 on the supporting member 2.
FIG. 7 illustrates an example of the structure of the double-row rotation type. The description below describes only the items that differ from the single-row rotation type, and the parts having the same structure will be omitted.
The double-row rotation type has two magnet support rings 4a and 4b which are arranged in parallel on the supporting member 2. While one magnet support ring 4a is immobilized and supported on the supporting member 2, the other magnet support ring 4b is supported so that it rotates freely via the bearing 3. On the outer circumference of the magnet support ring 4a on the immobilized side (referred to below as a fixed support ring) and on the magnet support ring 4b on the rotating side (referred to below as a rotating support ring) are arranged a plurality of permanent magnets 6a and 6b, as above. In addition, a plurality of pole pieces 7 are arranged as above, at equal intervals on the same circumference on a supporting member 8, and provided integrally with the supporting member 2, so as to face the outer surfaces of the group of permanent magnets 6a and 6b arranged on the fixed support ring 4a and the rotating support ring 4b. 
FIG. 6 (b) and FIG. 7 (b) illustrate the relative positions of the permanent magnets 6, 6a, and 6b and the pole piece 7 when in a braking state, and FIG. 6 (b) and FIG. 7 (c) illustrate the relative positions of the permanent magnets 6, 6a, and 6b and the pole piece 7 when in a non-braking state.
Large vehicles such as trucks and buses have a pre-installed compressed air source. Therefore, rotation of the magnet support ring 4 or the rotating support ring 4b is accomplished by an actuator such as an air cylinder via a yoke link 10 which protrudes from the side surface of the magnet support ring 4 or the rotating support ring 4b. 
The actuator utilizes a 3-position operating actuator which is able to switch the braking force in two stages by restricting the position of the permanent magnets to three positions. An example of the 3-position operating actuator is described below.
In Patent Reference 2, for example, a stepped piston formed on the inside of a cylinder opening to a small-diameter end surface is inserted into a large-diameter hole of a stepped cylinder with large-diameter holes and small-diameter holes formed on the inside. A rod which extends outward and which passes through an end wall on the large-diameter side of the stepped piston and an end wall of the large-diameter side of the stepped cylinder joins with the piston inserted into the cylinder, and a spring is disposed between the end wall of the stepped piston and the piston.
However, in the case of the actuator disclosed in Patent Reference 2, which uses a spring to achieve 3-position control, there is required a force powerful enough to compress the spring, in addition to requiring a force to change the position of the permanent magnet. Therefore, because a large force is needed and a large piston is needed, the device has to be increased in size, which makes it difficult to install in a vehicle. Moreover, since the cylinder and the piston are stepped, these parts are of a complex shape and therefore become costly to produce.
Accordingly, in order to solve the problems of Parent Reference 2, the present inventors proposed Patent Reference 3, which discloses a 3-position operating actuator which uses two cylinders.
Instead of using parts with complex shapes, as in the case of the actuator proposed in Patent Reference 2, the actuator proposed in Patent Reference 3 achieves 3-position control without increasing the size of the actuator beyond what is necessary.
However, since two cylinders are needed, the overall size of the device increases, the weight also increases, and it becomes difficult to install it in a vehicle. There was a further problem in that additional conduit was necessary to connect the two cylinders.